Abstract Within the Piedmont Uplands Section of southeastern Pennsylvania lies a metamorphic terrane containing the Peach Bottom Slate. The Peach Bottom Formation has been the center of attention for both quarrymen and geologists for more than 200 years. This probably early Paleozoic unit, underlying “Slate Ridge,” has been mined in Lancaster and York Counties, Pennsylvania, and Harford County, Maryland. The Peach Bottom Slate was judged the best building slate in the world at the 1850 World Exposition in London. Although mining terminated in the 1940s, the effect of the slate on the community and its heritage is well preserved today. The main purpose of the field trip is to examine some of main landmarks of the slate’s cultural effects, including a visit to a Welsh cemetery and a view of the district’s largest quarry. We also will seek an understanding of how the slate industry’s history has been preserved. Some problems of the regional geology also will be addressed. At our first two stops in Chester and Lancaster Counties, we will examine serpentinite within the Baltimore Mafic Complex. Our next stop in Lancaster County is a key exposure showing the relationship between the Peach Bottom Formation and its neighboring rock units. Despite much research on the structural implications of these rocks, the interpretation is still “up in the air.” Your opinions will be very much welcomed.

Abstract The Appalachian Piedmont in south-central Pennsylvania and north-central Maryland contains metasedimentary siliciclastic rocks (phyllites to quartzites) that were deposited largely offshore of Laurentia, prior to and during the early history of the Iapetan Ocean. The Peach Bottom area is centered on the belt of Peach Bottom Slate and overlying Cardiff Quartzite, which is surrounded by the late Neoproterozoic and early Paleozoic rocks of the Peters Creek and Scott Creek (new name) Formations. Their provenance was the Brandywine and Baltimore microcontinents that lay farther offshore of the Laurentian coast. This area also includes an ophiolitic mélange that formed in front of an advancing island arc in Iapetus. All these rocks lay largely undisturbed throughout much of the Paleozoic, experiencing only chlorite-grade greenschist facies metamorphism through deep burial. Alleghanian thrusting associated with the growth of the Tucquan anticline imparted their present widespread, monocline, steep southeast dip of the bed-parallel foliation.

Lititz Spring in southeastern Pennsylvania and a nearby domestic well were sampled for 9 months. Although both locations are connected to conduits (as evidenced by a tracer test), most of the year they were saturated with respect to calcite, which is more typical of matrix flow. Geochemical modeling (PHREEQC) was used to explain this apparent paradox and to infer changes in matrix and conduit contribution to flow. The saturation index varied from 0.5 to 0 most of the year, with a few samples in springtime dropping below saturation. The log P co 2 value varied from −2.5 to −1.7. Lower log P co 2 values (closer to the atmospheric value of −3.5) were observed when the solutions were at or above saturation with respect to calcite. In contrast, samples collected in the springtime had high P co 2 , low saturation indices, and high water levels. Geochemical modeling showed that when outgassing occurs from a water with initially high P co 2 , the saturation index of calcite increases. In the Lititz Spring area, the recharge water travels through the soil zone, where it picks up CO 2 from soil gas, and excess CO 2 subsequently is outgassed when this recharge water reaches the conduit. At times of high water level (pipe full), recharge with excess CO 2 enters the system but the outgassing does not occur. Instead the recharge causes dilution, reducing the calcite saturation index. Understanding the temporal and spatial variation in matrix and conduit flow in karst aquifers benefited here by geochemical modeling and calculation of P co 2 values.

Storms create stresses on karst systems that can alter the pathways and travel-times of water, solutes, and sediment. Flow contribution during storms is not only a matter of activation of new conduits, but is also a complex combination of water from conduits, enlarged fractures, and fractured matrix. In order to obtain evidence of pathway changes, we sampled three karst springs of varying size and maturity using data loggers for conductivity and water level, and storm water samplers for suspended sediment. The largest spring (Arch Spring) had the lowest conductivity of the three springs, indicating mainly conduit pathways at base flow. The high conductivity of base flow at the Nolte and Bushkill Springs pointed to contributions from slower-moving water in the fractured matrix. During storms, Arch Spring showed a consistent pattern of conductivity with a slight increase, then a large decrease, indicating an initial fracture flush of high-conductivity water, then passage of low-conductivity water from the precipitation. During storms, the conductivity of the middle-sized spring (Nolte Spring) either dropped immediately, or increased sharply then declined as storm water reached the spring. The smallest spring (Bushkill Spring) had a predictable conductivity pattern, with a sharp decrease and gradual recovery, suggesting shorter paths during storms than base flow. Sediment concentrations during storms were lowest at Nolte Spring and higher at Bushkill and Arch Springs, indicative of the fast flow through conduits or enlarged fractures suggested by the latter two springs during storms. The storm-water pathways vary from spring to spring and from storm to storm. These data show the importance of continuous monitoring to understand spring behavior.